This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Dynamic Nuclear Polarization (DNP), which has been well-known since the early years of magnetic resonance, is now becoming a powerful tool to improve the sensitivity and resolution for NMR and MRI techniques in structural biology and medicine. Transfer of polarization from unpaired electron spins to coupled nuclear spins (protons, 15N, 13C) results in an enhancement of the NMR signal. This enhancement could be as high as three orders of magnitude for solid state NMR. However, the current theoretical understanding for DNP is incomplete and ACERT, with our High Field ESR technology and expertise in ESR relaxation, is in a good position to better understand this phenomenon. Observation of DNP effects in magnetic fields routinely used for NMR experiments (3 [unreadable]20 Tesla) requires microwave irradiation of the sample at the corresponding High Field/High Frequency ESR range: 90 [unreadable]600 GHz. ACERT has a unique capability to carry out ESR experiments at a wide range of frequencies. Prof. McDermott's (Columbia University) interest in DNP is focused on membrane proteins with attached radicals. It has been recently found that nitroxide biradicals with relatively short interspin distances (d 10[unreadable]) are very efficient polarizing agents. Moreover, it has been shown that rigid biradicals are even more powerful polarizing agents than biradicals with flexible tethers. However, there is much that is not well understood about the biradicals and how they work for DNP. In particular, the exact mechanism of DNP relaxation enhancement by multiradicals is still unclear. A detailed study of ESR relaxation for existing and perspective DNP polarizing agents at high frequencies will provide valuable insights into the DNP phenomenon. To obtain DNP enhanced NMR signals from paramagnetic nuclei embedded into the hydrophobic core of the membrane, the nuclei should be accessible for the polarizing agent. ESR spectroscopy, and High Field ESR spectroscopy in particular, can easily determine the depth location of possible polarizing agents in the membrane and predict the accessibility of various transmembrane protein fragments.